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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 152
Registered: 4-2007
Posted on Wednesday, 11 July, 2018 - 08:49 am:   Edit PostDelete PostView Post/Check IP

Hi
Can anyone advise the optimum RPM at idle,for a Mk 6 manual, 4.5 litre engine
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Larry Kavanagh
Prolific User
Username: shadow_11

Post Number: 193
Registered: 5-2016
Posted on Wednesday, 11 July, 2018 - 11:25 am:   Edit PostDelete PostView Post/Check IP

I got the following from a Google search for Bentley MK VI RPM with Stromberg carbs:
1) Throttle stop screw should be set to 350RPM then 2) Slow running mixture screws should be turned individually by hand until the most even running is achieved, screwing in weakens the mixture while screwing out enriches it. Normal setting for slow running mixture screws is 1 1/4 turn back from fully in. I'm not familiar with these engines but hope the above is of some help.
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 823
Registered: 4-2015
Posted on Wednesday, 11 July, 2018 - 06:04 pm:   Edit PostDelete PostView Post/Check IP

Mark...
I would have to agree with Larry's data. I've never measured the idle rpm on any of mine but I am not surprised that it is that low. The 6 cylinder RR engines are capable of a very silent and slow tick over. You should barely realize that the engine is running with bonnet closed.

.
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christopher carnley
Unregistered guest
Posted From: 109.149.198.147
Posted on Wednesday, 11 July, 2018 - 06:51 pm:   Edit PostDelete PostView Post/Check IP

Only very few MK VI Bentleys invariably exports to the USA , were fitted with the Silver Dawn inlet manifold and carb, the vast majority have twin SU,s.
At low idle the tick over is "lumpy". The only car that I have ever experienced with a smooth idle was one where the owner had replaced the oil pump two teeth out of position. When the car refused to start, a friend of his re-located the the HT leads. 263514.

(Message approved by david_gore)
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 153
Registered: 4-2007
Posted on Wednesday, 11 July, 2018 - 08:57 pm:   Edit PostDelete PostView Post/Check IP

My car is fitted with twin SU, it idles well around 500 RPM, but I think this may be a little high, below this as you said Christopher Carnley it tend to get s little lumpy, trying to work out if this is just the mixture or a trait of the engine.
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Larry Kavanagh
Prolific User
Username: shadow_11

Post Number: 195
Registered: 5-2016
Posted on Wednesday, 11 July, 2018 - 10:06 pm:   Edit PostDelete PostView Post/Check IP

Have you tried synchronising the twin SUs using dial gauges in the dampers. Have a look at "British Tool Works". I recently purchased a set of damper adapters and dial gauges from Kelly Opfar there and am very pleased with the results, it's much more accurate than balancing by ear with a hose.
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 154
Registered: 4-2007
Posted on Friday, 13 July, 2018 - 01:48 pm:   Edit PostDelete PostView Post/Check IP

Thank you Larry, I will look into the guages.

If everthing is as it should be is 500 rpm a good idle speed or should it be lower.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1922
Registered: 9-2004
Posted on Friday, 13 July, 2018 - 04:48 pm:   Edit PostDelete PostView Post/Check IP

450RPM at idle is possible with the engine running smooth and quiet due to the heavy flywheel with all else with the carbs timing etc correct.
BTW the carbs are set on the more rich than weak side.
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Chris Browne
Prolific User
Username: chrisb

Post Number: 297
Registered: 2-2010
Posted on Saturday, 14 July, 2018 - 07:27 am:   Edit PostDelete PostView Post/Check IP

Hi all,
When David and I restored our 1958 Cloud 1, once carbs were rebuilt and the engine tuned very carefully, we achieved a perfectly smooth tick over at 450rpm as per the factory manual. You could barely hear the engine at tick over with the bonnets closed and we even did the old trick of balancing a one pound coin on the rocker cover - which stayed put!
Kind regards,
Chris
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 155
Registered: 4-2007
Posted on Saturday, 14 July, 2018 - 08:58 am:   Edit PostDelete PostView Post/Check IP

Thank you every one, I will aim for 450RPM, might need a bit of adjusting to get the engine smooth at this RPM
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Chris Browne
Prolific User
Username: chrisb

Post Number: 298
Registered: 2-2010
Posted on Saturday, 14 July, 2018 - 08:00 pm:   Edit PostDelete PostView Post/Check IP

Hi Mark,
It may take time to set everything up perfectly (it took us several attempts before we were happy with it) but the satisfaction of watching a quiet and lazy six humming away is more than worth the effort. Good luck with it. As an aside, we had an issue with the engine only firing on five cylinders which we eventually traced to a faulty, brand new NGK spark plug which we had only just fitted as a set of six. The moral to the story being that even if you fit new, branded parts, you may still have an unexpected failure. Learn to expect the unexpected.
Kind regards,
Chris
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 829
Registered: 4-2015
Posted on Sunday, 15 July, 2018 - 05:32 pm:   Edit PostDelete PostView Post/Check IP

Correct me if I err, but is it fair to say that unless the compressions of all the cylinders are equal, no amount of tinkering will offset or negate that imbalance created by the varying compressions as the engine rotates and fires, especially at low rpms? I assume that when an engine was new from the factory all the compressions were the same but as time, wear, and problems afflict the engine the compressions begin to change and nothing can be done about that condition other than careful rebuilding of rings and valves, etc?

.
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Chris Browne
Prolific User
Username: chrisb

Post Number: 299
Registered: 2-2010
Posted on Sunday, 15 July, 2018 - 06:54 pm:   Edit PostDelete PostView Post/Check IP

A fair point, Christian but I made the assumption that compression in each cylinder would have been checked for approximate balance. I think a variation of no more than 10 or 15% would be acceptable but anything over that on a worn engine would inevitably cause less than perfect running conditions.
Kind regards,

Chris
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 833
Registered: 4-2015
Posted on Monday, 16 July, 2018 - 10:15 am:   Edit PostDelete PostView Post/Check IP

Chris...
Perhaps what I was seeking is affirmation that unless I err, there is no way other than rebuilding to affect irregular compressions in pursuit of a more regular idle firing rhythm...or is there? Even when you rebuild and considering that each cylinder's compression is dependent on only the rings and the valves to retain that compression there is no way of knowing that all cylinder compressions will be equal until the head is back on and a test can be effected...or is there? I ask simply because the compressions on the Phantom II are 60,60,65,65,70,70. While this is not considerable variance, I suspect that it contributes to a bit...shallow as it may be...of irregular idle firing symmetry. Can anything be done about the lower compressions or no?

.
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Mark Taxis
Prolific User
Username: mark_taxis

Post Number: 156
Registered: 4-2007
Posted on Monday, 16 July, 2018 - 11:24 am:   Edit PostDelete PostView Post/Check IP

I haven't done a compression test for a while, might have a look at this first.
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 836
Registered: 4-2015
Posted on Monday, 16 July, 2018 - 04:00 pm:   Edit PostDelete PostView Post/Check IP

...and if they are not precisely the same, is there anything that can be done to adjust them or does it become a condition that you just have to live with barring an expensive attempted adjustment via rebuild with no certainty of success until the job is done and compressions tested again?

.
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Chris Browne
Prolific User
Username: chrisb

Post Number: 300
Registered: 2-2010
Posted on Monday, 16 July, 2018 - 07:44 pm:   Edit PostDelete PostView Post/Check IP

If the compression figures are outside of the tolerances, it points to valve issues or piston rings and there are various tests which can be done, before stripping down begins to determine where the problems lie. If you are unlucky enough to be in this situation, the choice is to live with it and set it up as best you can or bite the bullet and strip it down. It depends on your ambition and the depths of your pockets!
Kind regards,
Chris
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1929
Registered: 9-2004
Posted on Tuesday, 17 July, 2018 - 12:21 am:   Edit PostDelete PostView Post/Check IP

A great deal can be carried out, first check for any tight tappets, ie clearance.
Next add some t/stroke oil to fuel, ie for sticking piston rings.

Carry out a cylinder leakage test not a compression test for starters.
Christian did you not carry this out on the pre war Phantom!
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 837
Registered: 4-2015
Posted on Tuesday, 17 July, 2018 - 12:35 am:   Edit PostDelete PostView Post/Check IP

P.L.: I am not familiar with a leak down test. Please illuminate me. Does it check the speed of loss of compression in a tested cylinder, or what?

This was actually a theoretical question, i.e. is it likely that irregular but similar compressions as noted (not widely dissimilar or one almost zero) will cause a somewhat irregular idle firing rhythm? I would presume so. Yes, or no?

.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1930
Registered: 9-2004
Posted on Tuesday, 17 July, 2018 - 03:58 am:   Edit PostDelete PostView Post/Check IP

The leak down test will show a % drop of compressed air set pressure on each cylinder with engine locked at TDC.

Will show cracked head, head gasket blown by, valve seat failing, piston ring blow by, cracked liner,

If an engine has a really bad fault you can tell by the engine starter cranking sound with the king lead removed, if running the exhaust note will be irregular.
Piston blow by can be shown by oil misting fumes with the oil filler removed with engine running.
There are other points but will let someone with more brain power than me explain.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1932
Registered: 9-2004
Posted on Tuesday, 17 July, 2018 - 07:08 pm:   Edit PostDelete PostView Post/Check IP

And not forgetting the use of the whistle!
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 839
Registered: 4-2015
Posted on Wednesday, 18 July, 2018 - 05:48 pm:   Edit PostDelete PostView Post/Check IP

Just wondering the procedure for compression testing. I have always removed all spark plugs to alleviate all compression from all cylinders under the theory of making it easy for the starter to spin the engine. I then put the compression tester into a cylinder and use starter to spins engine a few revolutions and read compression.

Here is the question: Is it perhaps preferable to leave all the spark plugs in place and only substitute the compression tester for the plug in the cylinder being tested? In this later instance all the compressions are about similar (same as when engine is actually running) whereas in the spark plugs removed instance there is no compression on 5 of the 6 cylinders and then there is a jolt when the cylinder with compression cycles. Is it possible that this periodic jolt of compression (every 6th cylinder as they revolve) can give a false reading? Hope I'm being clear.

.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1933
Registered: 9-2004
Posted on Wednesday, 18 July, 2018 - 06:21 pm:   Edit PostDelete PostView Post/Check IP

How about opening the throttle with some testing!
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 840
Registered: 4-2015
Posted on Wednesday, 18 July, 2018 - 08:19 pm:   Edit PostDelete PostView Post/Check IP

What? You lost me. Unless I neglected to note that obviously either the main lead from coil is disconnected or the ignition is not in the "on" position. Well, "obviously" only if you realize (you had a 20/25 did you not?) that with RR and B chassis prior to the Cloud series there is a separate switch that turns on the ignition and no key is used to "start" the engine. The key ONLY locks/unlocks the switchbox. With an unlocked switchbox you simply turn "on" the main switch, turn "on" the ignition switch, and push the start button. If you do not turn the ignition "on", the start button (with main switch ON) will engage the starter, but the engine will not start. Maybe you are just kidding me?

Now why not answer the question? Should the spark plugs be in place, or removed when doing a compression test??

.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1934
Registered: 9-2004
Posted on Wednesday, 18 July, 2018 - 09:25 pm:   Edit PostDelete PostView Post/Check IP

Removed, the engine will crank more slowly with the plugs in giving a false reading.
With the engine spinning easy but if it starts slowing getting to the final cylinders, that will be a sign of the battery needing a charge or that will give you inconsistent results.

BTW for the elementary important bit.
Throttle open, normally.
It lets max air into the cylinders for full filling and for more accurate compression testing.


Up a gear, carry out a cylinder leakage test for
perfection!
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christopher carnley
Unregistered guest
Posted From: 86.169.76.167
Posted on Wednesday, 18 July, 2018 - 09:17 pm:   Edit PostDelete PostView Post/Check IP

REMOVE All PLUGS. OPEN THE THROTTLE TO LET THE AIR IN!

(Message approved by david_gore)
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1935
Registered: 9-2004
Posted on Wednesday, 18 July, 2018 - 10:51 pm:   Edit PostDelete PostView Post/Check IP

"REMOVE All PLUGS. OPEN THE THROTTLE TO LET THE AIR IN!"
Well spotted,
balanced is the operative left out word.

It lets max balanced air into the CYLINDER for full filling and for more accurate compression testing.
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 841
Registered: 4-2015
Posted on Thursday, 19 July, 2018 - 12:36 am:   Edit PostDelete PostView Post/Check IP

Well, I'm glad I asked.
Christopher...Thank you for clarification. I thought P.L. was pulling my leg and cryptically cautioning that with the spark plugs in place the engine would start...or something. Was not sure. Also, I think he means "Removed. Otherwise the engine will crank more slowly with the plugs in and give a false reading."
Anyway...never heard about the throttle being open to let air in issue. Yesterday I moved the PII back to storage for a few weeks while off onto a project in Montana but am now looking forward to redoing the test again just to see if the numbers change.

.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1936
Registered: 9-2004
Posted on Thursday, 19 July, 2018 - 06:00 am:   Edit PostDelete PostView Post/Check IP

Lets not forget the variations of cylinder pressure attributed to the cam shaft advance retard of a incorrectly fitted cam or worn lobes.

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